Control Valve "Fail-Safe" Positions

Cause of Fail-Safe Condition: Loss of Air Pressure

A. LINEAR SPRING/DIAPHRAGM ACTUATORS. Used with sliding stem control valves: i.e.
globe-style valves. Can be accomplished two ways:
1. Fixed seat ring/plug orientation. Springs are interchanged to either above or below actuator
diaphragm.

2. Fixed spring orientation. Plug and seat ring positions are reversed relative to each other. In
the Fail Open design, plug travel is above the valve seat. In the Fail Closed design, plug travel is
below the seat.

Rotary Spring/Diaphragm Actuators

Used with rotary control valves; i.e. butterfly, eccentric plug. Reversing the fail mode for
this type of valve is normally accomplished by reversing the location of lever arm and plug. In
order to maintain consistency, ATO-FC action will be considered as "Reverse" action for rotary
or sliding-stem control valves.
Control Valve ATC-FO Action
Control Valve ATO-FC Action
Benchset Range - ATO-FC
 Shutoff capability is directly proportional to the ability to "unload" the actuator down to a 0
psig pressure level. If there is any instrument that will NOT unload down to the 0 psig output
pneumatic pressure, the shutoff capability will be reduced. Devices that can cause a problem by
NOT unloading down to 0 psig are:
- I/P Transducer with a 3-15 psig output range; the device will only unload down to 2 -
2.5 psig. For the above actuator, the shutoff capability is "halved"; i.e. (2.5/5) x 600 psid
= 300 psid.
- Some pneumatic controllers.
A device that DOES unload down to 0 psig is a POSITIONER.

Benchset Range - ATC-FO

 Shutoff capability is directly proportional to the ability to "load" the actuator up to
approximately 18 psig pressure level. If there is any instrument that will NOT load up to the 18
psig output pneumatic pressure, the shutoff capability will be reduced. Devices that can cause a
problem by NOT loading up to the 18 psig level are:
- I/P Transducer with a 3-15 psig output range; the device may only load up to 16-17
psig level. for the above actuator, the shutoff capability is "reduced;" i.e. ((16.5-13)/5) x
600 = 420 psid.
- Some pneumatic controllers.
A device that DOES unload down to 0 psig is a POSITIONER.

Control Valves Do What They Are Told!

Being the Final Control Element in a system is not an easy job. To start with, you are
blamed for any and all problems that crop up in the process. You are subjected to corrosion,
high velocity, cavitation, flashing liquids, cryogenic temperatures, high temperatures, abrasion,
and thermal shock. You are expected not only to throttle along through all this, but most likely,
you are also being asked to act as a block valve and shut off tight.
As you work with control valves always keep in mind that a control valve only does what it
is told to do.
 A Control Valve is a power-operated device used to modify the fluid flow rate in a process
system. Well, what happens if the power is cut off? When a Control Valve is sized or selected
to do a particular job, one of the first questions you should consider is how that valve will
respond in the event of a loss of signal or power. This is called its "fail-safe mode" and
knowing the fail-safe mode is the key to troubleshooting it.
In most applications (about 80%), it is desirable for valves to fail closed. In other
applications, you might want a valve to fail open or fail in place. Safety concerns and process
requirements will mandate the fall mode of the valve.
When a valve is not sitting in its fail position, is is being told how and when to move by some
external signal.
By the comments one hears, you would be led to believe that control valves sit around and
think up things to do on their own. Perhaps this will some day be true when all control valves
are "smart."
If a Control Valve is observed in an unstable condition or appears to not be responding
correctly to an input signal, remember that something is telling the valve to behave that way.
A control valve is only as strong as its weakest link.
When the 1965 Ford Mustang first appeared, it was powered by a 6-cylinder engine with a
3-speed transmission - but it had a 140 m.p.h.(225 k.p.h.) speedometer. The fact that it had a
140 m.p.h.(225 k.p.h.) speedometer did not mean it could actually travel that fast. In the same
way, a control valve with a 600# rated valve body cannot throttle and shut off against 1440
pounds of pressure.
There are two basic types of control valves: rotary and linear. Linear-motion control valves
commonly have globe, gate, diaphragm, or pinch - type closures. Rotary-motion valves have
ball, butterfly, or plug closures. Each type of valve has its special generic features, which may,
in a given application, be either an advantage or a disadvantage.
Linear Valve Features
• TORTUOUS FLOW PATH
• LOW RECOVERY
• CAN THROTTLE SMALL FLOW RATES
• OFFERS VARIETY OF SPECIAL TRIM DESIGNS
• SUITED TO HIGH-PRESSURE APPLICATIONS
• USUALLY FLANGED OR THREADED

• SEPARABLE BONNET

Rotary Valve Features

• STREAMLINED FLOW PATH
• HIGH RECOVERY
• MORE CAPACITY
• LESS PACKING WEAR
• CAN HANDLE SLURRY AND ABRASIVES
• FLANGELESS
• INTEGRAL BONNET

• HIGH RANGEABILITY
In addition to linear and rotary, control valves are also classified according to their guiding
systems and the types of services they are used in.
Control Valve Classification
 Face to Face Dimensions

Control Valve Flow Characteristics

Trim design will affect how the valve capacity changes as the valve
moves through its complete travel. Because of the variation in trim design,
many valves are not linear in nature. THE RELATIONSHIP BETWEEN
VALVE CAPACITY AND VALVE TRAVEL IS KNOWN AS THE FLOW
CHARACTERISTIC OF THE VALVE. Valve trims are specially designed, or
characterized, in order to meet the large variety of control application needs.
This is necessary because most control loops have some inherent
nonlinearities, which you can compensate for when selecting control valve
trim.
Charts similar to Figure 1 (see below) are used to illustrate various
control valve flow characteristics. The percent of full flow through the valve is
plotted against valve stem position. The curves shown are typical of those
available from valve manufacturers. These curves are based on CONSTANT
PRESSURE DROP across the valve and are called INHERENT FLOW
CHARACTERISTICS.
The quick-opening characteristic provides large changes in flow for very
small changes in lift. It usually has too high a valve gain for use in
modulating control. So it is limited to on-off service, such as sequential
operation in either batch or semi-continuous processes.
The majority of control applications are valves with linear, equal-percentage,
or modified-flow characteristics.
• Linear - flow capacity increases linearly with valve travel.
• Equal percentage - flow capacity increases exponentially with valve
trim travel; equal increments of valve travel produce equal percentage
 changes in the existing Cv.

• A modified parabolic characteristic is approximately midway between

linear and equal-percentage characteristics. It provides fine throttling
at low flow capacity and approximately linear characteristics at higher
flow capacity.
When valves are installed with a pump, pipes, fittings, and other process
equipment, the pressure drop across the valve will vary as the plug moves
through its travel. When the actual flow in a system is plotted against valve
opening, the curve is called the INSTALLED FLOW CHARACTERISTIC.
Figure 1
Inherrent Flow Characteristics For Common Valve Trim Designs

"Tips & Tricks"

1. If you are dealing with a corrosive fluid, choose the valve body and trim
material to match the pump casing and impeller.
2. Velocity is the key to handling abrasive materials. Normal city water
velocity is about 7 to 10 F.P.S. (clean liquid). If you have a fluid that is
abrasive, keep the velocity as low as possible - without having the particles
drop out of suspension.
3. Always sense pressure where you want to control it. Many control valves
and pressure regulators do not function properly simply because they are
sensing pressure at one point and being asked to control it somewhere
else.
4. Velocity is the key to handling noise. Noise is energy. When dealing with
high pressure drop situations try always to keep the velocities below 0.3
mach. on the inlet pipe, valve body, and outlet pipe.
5. If you use a transducer in a control loop, specify a positioner on the valve.
Otherwise the transducer will rob the actuator of available thrust, and the
 valve will leak when it is supposed to shut off.
6. In cavitating fluids - even if the control valve has cavitation trim in it - be
sure to allow a straight run of downstream pipe after the valve. If there is a
pipe "T"or elbow immediately downstream, the flow will choke out and back
up into the valve.
7. If you use a control valve with a bellows seal in it, try to size the valve so
that its normal throttling position is near the bellows "at rest" position. This
will minimize wear on the bellows.
8. Don't use a valve below 10% of flow if at all possible. Even though a valve
may have good rangeability, if the valve is used in an abrasive or erosive
service (steam), it will not hold up unless it has hardened trim.
9. If a PLC is being used to control the valves in a system, specify the valves
with a linear flow characteristic.
10. If a control valve is started up and fails to respond - or goes to full open
or full closed and stays there - check the controller and reverse the
controller's action.
Troubleshooting
1. Start with the fail mode of the valve.
A: If the valve fails closed and is leaking...

1. Disconnect the positioner or controller input.

2. If the valve has a hand wheel, check to see that it is backed out.
3. Check to see if the bench range is correct.
4. Check to see if there is trash in, or damage to, the valve seat.

2. Next check the positioner.

3. Next check the controller.
Do not rely on the control room to generate signals. Generate your own with
equipment that you know is properly calibrated. Do not assume anything.
Remember that control valves only do what you tell them to. Many control
valve problems turn out to be a problem somewhere else

Control Valve Packing

Packing is a sealing system which normally consists of a deformable
material such as TFE, graphite, asbestos, Kalrez, etc. Usually the material is
in the form of solid or split rings contained in a packing box. Packing material
is compressed to provide an effective pressure seal between the fluid in the
valve body and the outside atmosphere.
At one time it was believed that the more packing you had in a control
valve the better it would seal. Since FUGITIVE EMISSIONS has become a
concern, extensive studies have been made which have shown that better
sealing can be obtained by minimizing the number of packing rings.
New standards are being developed to which manufacturers will be
asked to test their control valves. Test results from using these standards
will allow a user to predict with some certainty how well a particular valve
 and packing combination will hold up.
Definitions
Consolidation: Packing consolidation is the shortening of a packing stack
under load due to the elimination of voids in, between, and around the
packing rings. This causes a reduction in packing stress (Radial Load) and
consequently an increase in leakage. Consolidation can occur when the
packing wears, cold flows, is subjected to thermal gradients, or if a non-
uniform stress distribution in the packing exists.
Extrusion: When packing is loaded to its proper stress level it has a
tendency to cold flow and will extrude between the stem and the follower.
Any increase in temperature will increase the tendency of the packing to cold
flow. PTFE is very susceptible to this because the hotter it is the quicker it
will cold flow and because PTFE has an expansion rate roughly ten times
that of carbon steel. As the packing tries to expand in the fixed volume of the
packing gland, extrusion will occur. This material loss due to extrusion will
relieve the axial stress, which relaxes the radial stress and results in a loss
of seal.
Migration: Packing migration occurs when a portion of the packing is caught
by a rough stem and is removed from the packing box as the stem slides in
and out of the packing box. (Applies only to Linear Valves.)
Packing System Design Principles
1. In order to minimize stem friction and wear on the packing, the stem
surface finish should be in the 8 to 16 RMS range.
2. The stem of the valve should be held concentric with the packing bore.
This helps to uniformly compress the packing. This is best accomplished by
guiding the stem at the top and the bottom of the packing bore.
3. To minimize packing extrusion under load, the inner diameter of packing
spacers should be held as close to the stem diameter as possible. Anti-
extrusion washers can also be helpful in minimizing extrusion.
4. It is desirable to use a wiping mechanism. The stem-wiping device should
be at least a stroke distance away from the packing to prevent damage to
the stem and packing by dragging particles and deposits into the packing
area.
Live-Loaded Packing Arrangements
 Internal Live-Loading

External Live-Loading
Spring-Loaded Packing

Jammed Packing
The live-loading packing spring is replaced by a fixed spacer of the same
material as the trim material.
Dual Packing

Pressure inside the valve is alternately greater than or less than (i.e.
vacuum) ambient pressure.
Dual Packing With Leak-Off Connection

Valve has a 1/4" (6mm) NPT tapped opening on its bonnet. Complete
 with removable steel plug for all body materials. The opening is located
between primary and secondary packing sets when the valve is equipped
with dual packing.

Internally Pressurized Bellows Stem Seal

Although it is the most expensive way in which to seal off the bonnet
assembly from escaping fluids, it is also the most effective way of handling
lethal, toxic, explosive, and corrosive fluids. As we head toward "ZERO
EMISSIONS" control requirements, the bellows seal will become more
popular.

Direct-Acting and Reverse-Acting Positioners

The terms "direct" and "reverse" are frequently used when discussing
control valves, positioners, and controllers. While the definitions of direct and
reverse seem pretty straightforward, they cause quite a bit of confusion -
especially when split-ranging is done.
The key to working with control valves and controllers is to remember that
there must always be a balance maintained in the system. "Direct" and
"reverse" are kind of like "positive" and "negative" in that where you find one
you will usually find the other.
While control valve bodies and control valve actuators can be described
as being direct acting or reverse acting, thinking about such things when
working through a system problem only adds to the confusion. Therefore, it
is always best to consider the FAIL SAFE mode of the valve and simply let
the control valve be what it may be.
Positioners, 99% of the time, will usually mimic the input signal from the
controller. That is, they will be DIRECT ACTING.
Direct-Acting Positioner
Input Increases Input Decreases
Output Increases Output Decreases

Equals Equals
 Increasing Signal Increasing Output Decreasing
Decreasing
from Controller from Positioner Signal
Output From
From
Positioner
Controller
Another reason the direct-acting pneumatic positioner is so popular is
that it can be by-passed and the control valve will respond to the input signal
from the controller as though the positioner were in the control loop. If a
positioner malfunction occurs or if the positioner causes the control valve to
become unstable, it can be easily by-passed. Many control valves in the field
are operating with a by-passed positioner.
Reverse-acting positioners are sometimes used on control valves, but
their appearance is rare. Occasionally one will be found in a split-ranging
sequence.
Reverse-Acting Positioner
Input Increases Input Decreases
Output Decreases Output Increases

Equals Equals

Increasing Signal Decreasing Output Decreasing

Increasing
from Controller from Positioner Signal
Output From
From
Positioner
Controller
Direct-Acting and Reverse-Acting Controllers
Controllers can be set up in either direct or reverse modes. It was stated
that 99% of the positioners are direct acting, and it follows that if a balance is
to be maintained in the control loop that 99% of the controllers will be
reverse acting. If the control valve and its controller are not in balance, the
control valve will either go to the wide-open position and stay there, or it will
stay closed and act as though it is not responding. This situation can
normally be corrected by reversing the action of the controller.
Direct-Acting Controller
Setpoint Increases Setpoint Decreases
Output Increases Output Decreases

Equals Equals

Increase Increase Decrease Decrease

in in in in
Setpoint Output Setpoint Output
Reverse-Acting Controller
Setpoint Increases Setpoint Decreases
Output Decreases Output Increases

Equals Equals

Increase Decrease Decrease Increase

 in in in in
Setpoint Output Setpoint Output

Two of the more common control valve uses are for pressure control. In
both instances, the controllers are reverse acting. Most pressure-reducing
valves will be fail-closed and most back-pressure control valves will be fail-
open. If the pressure-reducing valve were fail-open or the back-pressure
valve fail-closed, then the controllers would have been direct acting.

Valve Positioners
A valve Positioner is a device used to increase or decrease the air
pressure operating the actuator until the valve stem reaches the position
called for by the instrument controller.
Positioners are generally mounted on the side or top of the actuator.
They are connected mechanically to the valve stem so that stem position
can be compared with the position dictated by the controller.
A positioner is a type of air relay which is used between the controller
output and the valve diaphragm. The positioner acts to overcome hysteresis,
packing box friction, and valve plug unbalance due to pressure drop. It
assures exact positioning of the valve stem in accordance with the controller
output.
Reasons To Use Positioners
Increase control system resolution: i.e. fine control.
Allow use of characteristic cams.
Minimize packing friction effects: i.e. high-temperature packing.
Negate flow-induced reactions to higher pressure drops.
Increase speed of response to a change in process.
Allow split ranging.
Overcome seating friction in rotary valves.
Allow distances between controller and control valve.
Allow wide range of flow variation: i.e. operate at less than 10% travel under
normal conditions.
Allow increased usage of 4-20 mA electronic signal.
Increase fast venting (unloading) capability.
Permit use of piston actuators.
Facilitate operation when the higher number in the bench-set range is
greater than 15 psig: i.e. 10-30 psig, 6-30 psig, etc.
How Positioners Work
Although there are many different types of positioners, the basic
principles of operation are similar for all of them.
Principle of Operation:
 The positioner is mechanically connected to the stem of the valve. This
stem position is compared with the position called for by the instrument
controller, i.e. by the instrument output air signal. A separate air supply is
brought into the positioner for positioning the valve at exactly the point called
for by the controller.

Seat Leakage Classifications

Rule of Thumb:
There is no such thing as "Bubble Tight."
Control valves are designed to throttle. However, this is not a perfect
world, and control valves are also usually expected to provide some type of
shut-off capability. A control valve's ability to shut off has to do with many
factors. The type of valves for instance. A double-seated control valve will
usually have very poor shut-off capability. The guiding, seat material,
actuator thrust, pressure drop, and the type of fluid can all play a part in how
well a particular control valve shuts off.
There are actually six different seat leakage classifications as defined by
ANSI/FCI 70-2-1976. But for the most part you will be concerned with just
two of them: CLASS IV and CLASS VI. CLASS IV is also known as METAL
TO METAL. It is the kind of leakage rate you can expect from a valve with a
metal plug and metal seat. CLASS VI is known as a SOFT SEAT
classification. SOFT SEAT VALVES are those where either the plug or seat
or both are made from some kind of composition material such as Teflon.
Valve Leakage Classifications
Class I. Identical to Class II, III, and IV in construction and design intent, but
no actual shop test is made.
Class II. Intended for double-port or balanced singe-port valves with a metal
piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the
test fluid. Allowable leakage is 0.5% of the rated full open capacity.
Class III. Intended for the same types of valves as in Class II. Allowable
leakage is limited to 0.1% of rated valve capacity.
Class IV. Intended for single-port and balanced single-port valves with extra-
tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01%
of rated valve capacity.
Class V. Intended for the same types of valves as Class IV. The test fluid is
water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10
ml per minute per inch of orifice diameter per psi differential.
Class VI. Intended for resilient-seating valves. The test fluid is air or
nitrogen. Pressure is the lesser of 50 psig or operating pressure. The
leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per
minute for valve sizes 1 through 8 inches.

Nominal Port Diameter Allowable Leakage

(Inches) (ml Per Minute) (*Bubbles Per Minute)
1 0.15 1
1.5 0.30 2
2 0.45 3
2.5 0.60 4
3 0.90 6
4 1.70 11
6 4.00 27
8 6.75 45
10 9.00 63
12 11.5 81
*Bubbles per minute as tabulated are a suggested alternative based on a
suitable calibrated measuring device, in this case a 0.25-inch O.D. X 0.032-
inch wall tube submerged in water to a depth of from 1/8 to 1/4 inch. The
tube end shall be cut square and smooth with no chamfers or burrs. The
tube axis shall be perpendicular to the surface of the water. Other measuring
devices may be constructed and the number of bubbles per minute may
differ from those shown as long as they correctly indicate the flow in
milliliters per minute.
Note: Provisions should be made to avoid overpressuring of measuring
devices resulting from inadvertent opening of the valve plug.
Taken from ANSI B16.104-1976
 Cashco Terminology
ACTUATOR: A fluid-powered or electrically powered device that supplies
force and motion to a VALVE CLOSURE MEMBER.
AIR SET: Also SUPPLY PRESSURE REGULATOR. A device used to
reduce plant air supply to valve POSITIONERS and other control equipment.
Common reduced air supply pressures are 20 and 35 psig.
AIR-TO-CLOSE: An increase in air pressure to the ACTUATOR is required
to cause the valve to close. This is another way of saying the valve is Fail
Open or Normally Open.
AIR-TO-OPEN: An increase in air pressure to the ACTUATOR is required to
cause the valve to open. This is another way of saying the valve is FAIL
CLOSED or NORMALLY CLOSED.
ANSI: An abbreviation for the American National Standards Institute.
ANTI-CAVITATION TRIM: A special trim used in CONTROL VALVES to
stage the pressure drop through the valve, which will either prevent the
CAVITATION from occurring or direct the bubbles that are formed to the
center of the flow stream away from the valve BODY and TRIM. This is
usually accomplished by causing the fluid to travel along a torturous path or
through successively smaller orifices or a combination of both.
API: An abbreviation for the American Petroleum Institute.
ASME: An abbreviation for the American Society of Mechanical Engineers.
ASTM: An abbreviation for the American Society for Testing and Materials.
BALANCED TRIM: A trim arrangement that tends to equalize the pressure
above and below the valve plug to minimize the net static and dynamic fluid
flow forces acting along the axis of the stem of a GLOBE VALVE. Some
regulators also use this design, particularly in high pressure service.
BELLOWS SEAL BONNET: A BONNET which uses a BELLOWS for sealing
against leakage around the valve plug stem.
BENCH SET: The proper definition for bench set is the INHERENT
DIAPHRAGM PRESSURE RANGE, which is the high and low values of
pressure applied to the diaphragm to produce rated valve plug travel with
atmospheric pressure in the valve body. This test is often performed on a
work bench in the instrument shop prior to placing the valve into service and
is thus known as Bench Set.
BODY: The body of the valve is the main pressure boundary. It provides the
pipe connecting ends and the fluid flow passageway. It can also support the
seating surface and the valve CLOSURE MEMBER.
BONNET: The bonnet or bonnet assembiy is that portion of the valve
pressure retaining boundary which may guide the stem and contains the
PACKING BOX and STEM SEAL. The bonnet may be integral to the valve
body or bolted or screwed. The bonnet, if it is detachable, will generally
provide the opening to the valve body cavity for removal and replacement of
the internal TRIM. The bonnet is generally the means by which the actuator
is connected to the valve body.
BOOSTER: A pneumatic relay that is used to reduce the time lag in
pneumatic circuits by reproducing pneumatic signals with high-volume and
or high-pressure output. These units may act as volume boosters or as
amplifiers. A 1:2 booster will take a 3 to 15 psig input signal and output a 6
to 30 psig signal. It has also been shown that a booster may improve the
performance of a control valve by replacing a positioner. It can provide the
same stroking speed and can isolate the controller from the large capacitive
load of the actuator.
BUBBLE TIGHT: A commonly used term to describe the ability of a control
valve or regulator to shut off completely against any pressure on any fluid.
Unfortunately, it is completely unrealistic. Control valves are tested to ANSI
B16.104 and FCI 70-2-1976 which is the American National Standard for
Control Valve Seat Leakage. This standard uses 6 different classifications to
describe the valves seat leakage capabilities. The most stringent of these is
Class VI which allows a number of bubbles per minute leakage, depending
on the port size of the valve. The correct response to the question "Will that
valve go "Bubble Tight"? is to say this valve is tested to meet Class VI
shutoff requirements.
BUTTERFLY VALVE: A valve with a circular body and a rotary motion disk
closure member which is pivotally supported by its stem. Butterfly valves
come in various styles including eccentric and high-performance valves.
Butterfly valves are HIGH RECOVERY valves and thus tend to induce
CAVITATION in liquid services at much lower pressure drops and fluid
temperatures than the globe style valve. Due to instability problems with the
older design butterfly valves, many people will limit the travel of the valve at
60 degrees of rotation on throttling services. This can also help keep the
valve out of CAVITATION problems.
CAGE: A hollow cylindrical trim element that is sometimes used as a guide
to align the movement of a VALVE PLUG with a SEAT RING. It may also act
to retain the seat ring in the valve body. On some types of valves, the cage
may contain different shaped openings which act to characterize the flow
through the valve. The cage may also act as a NOISE ATTENUATION or
ANTI-CAVITATION device.
CAGE GUIDED VALVE: A type of GLOBE STYLE valve trim where the
valve plugs with the seat.
CAVITATION: Occurs only in liquid service. In its simplest terms cavitation is
the two-stage process of vaporization and condensation of a liquid.
Vaporization is simply the boiling of a liquid, which is also known as
FLASHING. In a control valve this vaporization takes place because the
pressure of the liquid is lowered, instead of the more common occurrence
where the temperature is raised. As fluid passes through a valve just
downstream of the orifice area, there is an increase in velocity or kinetic
energy that is accompanied by a substantial decrease in pressure or
potential energy. This occurs in an area called the VENA CONTRACTA. If
the pressure in this area falls below that of the vapor pressure of the flowing
fluid, vaporization (boiling) occurs. Vapor bubbles then continue downstream
where the velocity of the fluid begins to slow and the pressure in the fluid
recovers. The vapor bubbles then collapse or implode. Cavitation can cause
a Choked Flow condition to occur and can cause mechanical damage to
valves and piping.
CHOKED FLOW: Also known as CRITICAL FLOW. This condition exists
when at a fixed upstream pressure the flow cannot be further increased by
lowering the downstream pressure. This condition can occur in gas, steam,
or liquid services. Fluids flow through a valve because of a difference in
pressure between the inlet (Pl) and outlet (P2) of the valve. This pressure
difference (Delta-P) or pressure drop isessential to moving the fluid. Flow is
proportional to the square root of the pressure drop. Which means that the
higher the pressure drop is the more fluid can be moved through the valve. If
the inlet pressure to a valve remains constant, then the differential pressure
can only be increased by lowering the outlet pressure. For gases and steam,
which are compressible fluids, the maximum velocity of the fluid through the
valve is limited by the velocity of the propagation of a pressure wave which
travels at the speed of sound in the fluid. If the pressure drop is sufficiently
high, the velocity in the flow stream at the VENA CONTRACTA will reach the
velocity of sound. Further decrease in the outlet pressure will not be felt
upstream because the pressure wave can only travel at sonic velocity and
the signal will never translate upstream. Choked Flow can also occur in
liquids but only if the fluid is in a FLASHING or CAVITATING condition. The
vapor bubbles block or choke the flow and prevent the valve from passing
more flow by lowering the outlet pressure to increase the pres-sure drop. A
good Rule Of Thumb on Gases and Steam service is that if the pressure
drop across the valve equals or exceeds one half the absolute inlet
pressure, then there is a good chance for a choked flow condition.
Example:
P1 100 psig
P2 25 psig
_________
Delta P = 75
P1 (ABS) = 100 + 14.7 or 114.7 1/2 of 114.7 = 57.35
Actual pressure drop = 75
Choked Flow is probable.
The style of valve (that is whether it is a HIGH RECOVERY or a LOW
RECOVERY style) will also have an effect on the point at which a choked
flow condition will occur.
CLOSURE MEMBER: The movable part of the valve which is positioned in
the flow path to modify the rate of flow through the valve. Some of the
different types of closure members are the Ball, Disk, Gate, and Plug.
COEFFICIENT FLOW: A constant (Cv) that is used to predict the flow rate
through a valve. It is related to the geometry of the valve at a given valve
opening. See Cv.
CONTROL VALVE: Also known as the FINAL CONTROL ELEMENT. A
power-operated device used to modify the fluid flow rate in a process control
system. It usually consists of a BODY or VALVE and an ACTUATOR, which
responds to a signal from the controlling system and changes the position of
a FLOW CONTROLLING ELEMENT in the valve.
CONTROL VALVE GAIN: The relationship between valve travel and the flow
rate through the valve. It is described by means of a curve on a graph
expressed as an INSTALLED OR INHERENT CHARACTERISTIC.
CONTROLLER: A device which tells a CONTROL VALVE what to do.
Controllers can be either pneumatic or electronic. There are pressure,
temperature, ph, level, differential, and flow controllers. The job of the
controller is to sense one of the above variables and compare it to a set
point that has been established. The controller then outputs a signal either
pneumatic or electronic to the control valve, which then responds so as to
bring the process variable to the desired set point.
CRITICAL FLOW: See the definition for CHOKED FLOW.
CV: The VALVE FLOW COEFFICIENT is the number of U.S. gallons per
minute of 60 degree F water that will flow through a valve at a specified
opening with a pressure drop of 1 psi across the valve.
DELTA-P: Differential Pressure. The inlet pressure (Pl) minus the outlet
pressure (P2).
Example:
P1 = 100 psig
P2 = 25 psig.
___________
Delta-P = 75
DIAPHRAGM: A flexible pressure-responsive element that transmits force to
the diaphragm plate and actuator stem.
DIAPHRAGM ACTUATOR: Is a fluid (usually pneumatic) pressure-operated,
spring-opposed diaphragm assembly which positions the valve stem in
response to an input signal.
DIAPHRAGM PRESSURE: See Bench Set.
DIAPHRAGM VALVE: A valve with a flexible linear motion CLOSURE
MEMBER that is forced into the internal flow passageway of the BODY by
the ACTUATOR. Pinch or Clamp valves and Weir-type valves fall into this
category.
DIRECT ACTING: This term has several different meanings depending upon
the device it is describing. A DIRECT-ACTING ACTUATOR is one in which
the actuator stem extends with an increase in diaphragm pressure. A
DIRECT-ACTING VALVE is one with a PUSH-DOWN-TO-C LOSE plug and
seat orientation. A DIRECT-ACTING POSITIONER or a DIRECT-ACTING
CONTROLLER outputs an increase in signal in response to an increase in
set point.
DIRECT ACTUATOR: Is one in which the actuator stem extends with an
increase in diaphragm pressure.
DUAL SEATING: A valve is said to have dual seating when it uses a resilient
or composition material such as TFE, Kel-F, or Buna-N, etc. for its primary
seal and a metal-to-metal seat as a secondary seal. The idea is that the
primary seal will provide tight shut-off Class VI and if it is damaged the
secondary seal will backup the primary seal with Class IV shut-off.
DYNAMIC UNBALANCE: The total force produced on the valve plug in any
stated open position by the fluid pressure acting upon it. The particular style
of valve, i.e. single-ported, double-ported, flow-to-open, flow-to-close, has an
effect on the amount of dynamic unbalance.
EFFECTIVE AREA: For a DIAPHRAGM ACTUATOR, the effective area is
that part of the diaphragm area that is effective in producing a stem force.
Usually the effective area will change as the valve is stroked - being at a
maximum at the start and at a minimum at the end of the travel range. Flat
sheet diaphragms are most affected by this; while molded diaphragms will
improve the actuator performance, and a rolling diaphragm will provide a
constant stem force throughout the entire stroke of the valve.
ELECTRIC ACTUATOR: Also known as an Electro-Mechanical Actuator
uses an electrically operated motor-driven gear train or screw to position the
actuator stem. The actuator may respond to either a digital or analog
electrical signal.
END CONNECTION: The configuration provided to make a pressure-tight
joint to the pipe carrying the fluid to be controlled. The most common of
these connections are threaded, flanged, or welded.
EQUAL PERCENTAGE: A term used to describe a type of valve flow
characteristic where for equal increments of valve plug travel the change in
flow rate with respect to travel may be expressed as a constant percent of
the flow rate at the time of the change. The change in flow rate observed
with respect to travel will be relatively small when the valve plug is near its
seat and relatively high when the valve plug is nearly wide open.
EXTENSION BONNET: A bonnet with a packing box that is extended above
the body to bonnet connection so as to maintain the temperature of the
packing above (cryogenic service) or below (high-temp service) the
temperature of the process fluid. The length of the extension depends on the
amount of temperature differential that exists between the process fluid and
the packing design temperature.
FACE-TO-FACE: Is the distance between the face of the inlet opening and
the face of the outlet opening of a valve or fitting. These dimensions are
governed by ANSI/ISA specifications.
The following Uniform Face-to Face Dimensions apply.
SPECIFICATION VALVE TYPE
ANSI/ISA S75.03 INTEGRAL FLANGED GLOBE STYLE CONTROL
VALVES
ANSI/ISA S75.04 FLANGELESS CONTROL VALVES ANSUISA S75.20
SEPARABLE FLANGE GLOBE STYLE CONTROL VALVES
FAIL-CLOSED: Or NORMALLY CLOSED. Another way of describing an
AIR-TO-OPEN actuator. Approximately 80% of all spring return diaphragm
operators in the field are of this construction.
FAIL-IN-PLACE: A term used to describe the ability of an actuator to stay at
the same percent of travel it was in when it lost its air supply. On SPRING
RETURN ACTUATORS this is accomplished by means of a LOCK-UP
VALVE. On PISTON ACTUATORS a series of compressed air cylinders
must be employed.
FAIL-OPEN: Or NORMALLY OPEN. Another way of describing an AIR-TO-
CLOSE actuator.
FAIL-SAFE: A term used to describe the desired failure position of a control
valve. It could FAIL-CLOSED, FAIL-OPEN, or FAIL-IN-PLACE. For a spring-
return operator to fail-in-place usually requires the use of a lock-up valve.
FEEDBACK SIGNAL: The return signal that results from a measurement of
the directly controlled variable. An example would be where a control valve
is equipped with a positioner. The return signal is usually a mechanical
indication of valve plug stem position which is fed back into the positioner.
F1: Or PRESSURE RECOVERY FACTOR. A number used to describe the
ratio between the pressure recovery after the VENA CONTRACTA and the
pressure drop at the vena contracta. It is a measure of the amount of
pressure recovered between the vena contracta and the valve outlet. Some
manufacturers use the therm Km to describe the pressure recovery factor.
This number will be high (0.9) for a GLOBE STYLE VALVE with a torturous
follow path and lower (0.8 to 0.6) for a ROTARY STYLE VALVE with a
streamlined flow path. On most rotary products the F1 factor will vary with
the degree of opening of the VALVE CLOSURE MEMBER. Note! F1 does
not equal Km.
FLANGELESS: A valve that does not have integral line flanges. This type of
valve is sometimes referred to as a Wafer Style valve. The valve is installed
by bolting it between the companion flanges with a set of bolts or studs
called line bolting. Care should be taken that strain-hardened bolts and nuts
are used in lieu of all-thread, which can stretch when subjected to tempera-
ture cycling.
FLANGELESS BODY: See FLANGELESS for a definition. This type of valve
is very economical from a manufacturing and stocking standpoint because a
valve that is rated as a 600# ANSI valve can also be used between 150#
and 300# ANSI flanges thus eliminating the need to manufacture three
different valve bodies or stock three different valve bodies. The down side is
that valves with flangeless bodies are not acceptable in certain applications -
particularly in refinery processes.
FLASHING: Is the boiling or vaporizing of a liquid. See the definition of
CAVITATION. When the vapor pressure downstream of a control valve is
less than the upsteam vapor pressure, part of the liquid changes to a vapor
and remains as a vapor unless the downstream pressure recovers
significantly, in which case CAVITATION occurs. Flashing will normally
cause a CHOKED FLOW condition to occur. In addition the vapor bubbles
can also cause mechanical damage to the valve and piping system.
FLOW CHARACTERISTIC: The relationship between valve capacity and
valve travel. It is usually expressed graphically in the form of a curve.
CONTROL VALVES have two types of characteristics INHERENT and
INSTALLED. The INHERENT characteristic is derived from testing the valve
with water as the fluid and a constant pressure drop across the valve. When
valves are installed into a system with pumps, pipes, and fittings, the
pressure dropped across the valve will vary with the travel. When the actual
flow in a system is plotted against valve opening, the curve is known as the
INSTALLED flow characteristic. Valves can be characterized by shaping the
plugs, orifices, or cages to produce a particular curve. Valves are
characterized in order to try to alter the valve gain.
Valve gain is the flow change divided by the control signal change. This is
done in an effort to compensate for nonlinearities in the control loop.
FLOW COEFFICIENT: See the definition for Cv.
GAIN: The relationship of input to output. If the full range of the input is equal
to the full range of the output, then the gain is 1. Gain is another way to
describe the sensitivity of a device.
GLOBE VALVE: A valve with a linear motion, push-pull stem, whose one or
more ports and body are distinguished by a globular shaped cavity around
the port region. This type of valve is characterized by a torturous flow path
and is also referred to as a LOW RECOVERY VALVE because some of the
energy in the flow stream is dissipated; and the inlet pressure will not
recover to the extent that it would in a more streamlined HIGH RECOVERY
VALVE.
HANDWHEEL: A manual override device used to stroke a valve or limit its
travel. The handwheel is sometimes referred to as a hand jack. It may be top
mounted, side mounted, in-yoke mounted or shaft mounted and
declutchable.
HARD FACING: A material that is harder than the surface to which it is
applied. It is normally used to resist fluid erosion or to reduce the chance of
galling between moving parts. Hard facing may be applied by fusion welding,
diffusion, or spray coating the material. Alloy #6 or Stellite is a common
material used for this purpose.
HARDNESS: A property of metals that is discussed frequently when
speaking of various component parts used in valve construction, particularly
valve trim. There are two hardness scales which are commonly used,
Rockwell & Brinell.

HARDNESS COMPARISON
ROCKWELL BRINELL
316 SST 76B 137
17-4 PH 34-38C 352
Hardened Inconel X-750
38-42C 401
#6 Stellite (Alloy 6) 40-44C 415
Chrome Plating 59-67C 725
Note that 316 SST is on the Rockwell B scale which means it is a much
softer material than the others shown.
HIGH RECOVERY VALVE: A valve design that dissipates relatively little flow
stream energy due to streamlined internal contours and minimal flow
turbulence. Therefore, pressure down stream of the valve VENA
CONTRACTA recovers to a high percentage of its inlet value. These types
of valves are identifiable by their straight-th rough flow paths. Examples are
most rotary control valves, such as the eccentric plug, butterfly, and ball
valve.
HYSTERESIS: The difference between up-scale and down-scale results in
instrument response when subjected to the same input approached from the
opposite direction. Example: A control valve has a stroke of 1.0 inch and we
give the valve a 9 psig signal. The valve travels 0.500 of an inch. We then
give the valve a 12 psig signal, and the valve travels to 0.750 of an inch.
When the valve is then given a 9 psig signal, the stroke is measured at
0.501. That represents hysteresis. Hysteresis can be caused by a multitude
of variables, packing friction, loose linkage, pressure drop, etc. If someone
asks you what the hysteresis of your control valve is, it is a bum question
because hysteresis is more aptly applied to an instrument than to a control
valve. There are simply too many variables in the valve and the system to
answer the question properly. The control valve only responds to the
controller signal and will move to a position to satisfy the controller - thus
negating the effects of hysteresis.
INCIPIENT CAVITATION: Is a term used to describe the early stages of
CAVITATION. At this point the bubbles are small, and the noise is more of a
hiss, like the sound of frying bacon. There is normally no mechanical
damage associated with incipient cavitation although it could have an effect
on the corrosive properties of some fluids.
INHERENT DIAPHRAGM PRESSURE: The high and low values of pressure
applied to the diaphragm to produce rated valve plug travel with atmospheric
pressure in the valve body. This is more commonly referred to as BENCH
SET.
INHERENT FLOW CHARACTERISTIC: It is the relationship between valve
capacity and valve travel and is usually expressed graphically. It is derived
from testing a valve with water as the fluid and with a constant pressure drop
across the valve. The most common types of inherent flow characteristics
are LINEAR, EQUAL PERCENTAGE, MODIFIED PARABOLIC, and QUICK
OPENING.
INSTALLED DIAPHRAGM PRESSURE: The high and low values of
pressure applied to the diaphragm to produce rated travel with stated
conditions in the valve body. The "stated conditions" referred to here mean
the actual pressure drops at operating conditions. Example: A control valve
may have an INHERENT DIAPHRAGM PRESSURE or BENCH SET of 8 to
15 psig. But when subjected to a 600 psig. inlet pressure, it may start to
open at 3 psig. and be full open at 15 psig. It is because of the forces acting
on the valve plug and the direction of flow through the valve (FLOW-TO-
OPEN or FLOW-TO-CLOSE) that the installed diaphragm pressure will differ
from the inherent diaphragm pressure.
INSTALLED FLOW CHARACTERISTIC: The flow characteristic when the
pressure drop across the valve varies with flow and related conditions in the
system in which the valve is installed. The purpose of characterizing a
control valve is to help compensate for nonlinearities in the control loop.
INSTRUMENT PRESSURE: The output pressure from an automatic
controller that is used to operate a control valve. It is the input signal to the
valve.
INTEGRAL SEAT: The flow control orifice and seat that is an integral part of
the valve body or cage. The seat is machined directly out of the valve body
and is normally not replaceable without replacing the body itself - although
some can be repaired by welding and remachining.
INTEGRAL FLANGE: A valve body whose flange connection is an integral or
cast part of the body. Valves with integral flanges were traditionally known to
have the ANSI short FACE-TO-FACE dimension ANSI/ISA S75.03. However
many manufacturers now produce valve bodies with both integral and
SEPARABLE FLANGES that will meet both the ANSI short and long face-to-
face dimensions.
I/P: An abbreviation for current-to-pneumatic signal conversion. This term is
commonly used to describe a type of transducer that converts an electric (4-
20 m.a) input signal to a pneumatic (3-15 psig.) output signal.
LANTERN RING: A rigid spacer used in the packing with packing above and
below it. The lantern ring is used to allow lubrication to the packing or allow
access to a leak off connection. On some of the new fugitive emission
packing systems, it also acts as a stem guide.
LAPPED-IN: A term that describes a procedure for reducing the leakage rate
on metal-to-metal seated valves and regulators. The plug and seat are
lapped together with the aid of an abrasive compound in an effort to
establish a better seating surface than would normally be achieved by
means of machining.
LEAKAGE CLASSIFICATION: A term used to describe certain standardized
testing procedures for CONTROL VALVES with a FLOW COEFFICIENT
greater then 0. 1 (Cv). These procedures are outlined in ANSI Standard d
B16.104-1976, which gives specific tests and tolerances for six seat leakage
classifications. It should be remembered that these tests are used to
establish uniform acceptance standards for manufacturing quality and are
not meant to be used to estimate leakage under actual working conditions.
Nor should anyone expect these leakage rates to be maintained after a
valve is placed in service. There is no standard test for SELF-CONTAINED
REGULATORS at this time. Note! You will see many instances where
regulators are specified using the above criteria.
LEAK-OFF: A term used to describe a threaded connection located on the
BONNET of a valve that allows for the detection of leakage of the process
fluid past the packing area.
LINEAR FLOW CHARACTERISTIC: A characteristic where flow capacity or
(Cv) increases linearly with valve travel. Flow is directly proportional to valve
travel. This is the preferred valve characteristic for a control valve that is
being used with a distributive control system (DCS) or programmable logic
controller (PLC).
LINEAR VALVE: Another name for a GLOBE VALVE. It refers to the linear
or straight-line movement of the plug and stem.
LIQUID PRESSURE RECOVERY: See (F1).
LOADING PRESSURE: The pressure used to position a pneumatic actuator.
It is the pressure that is actually applied to the actuator diaphragm or piston.
It can be the INSTRUMENT PRESSURE if a valve positioner is not used or
is bypassed.
LOCK-UP VALVE: A special type of regulator that is installed between the
valve POSITIONER and the valve ACTUATOR, where it senses the supply
air pressure. If that pressure falls below a certain level, it locks or traps the
air loaded into the actuator causing the valve to FAIL-IN-PLACE.
LOW RECOVERY VALVE: A valve design that dissipates a considerable
amount of flow stream energy due to turbulence created by the contours of
the flow path. Consequently, pressure downstream of the valve VENA
CONTRACTA recovers to a lesser percentage of its inlet value than a valve
with a more streamlined flow path. The conventional GLOBE STYLE control
valve is in this category.
MODIFIED PARABOLIC: A FLOW CHARACTERISTIC that lies somewhere
between LINEAR and EQUAL PERCENTAGE. It provides fine throttling at
low flow capacity and an approximately linear characteristic at higher flow
capacities.
NORMALLY CLOSED: See AIR-TO-OPEN.
NORMALLY OPEN: See AIR-TO-CLOSE.
P1: Is used to designate Inlet Pressure.
P2: Is used to designate Outlet Pressure.
PACKING: A sealing system that normally consists of a deformable material
such as TFE, graphite, asbestos, etc. It is usually in the form of solid or split
rings contained in a PACKING BOX that are compressed so as to provide an
effective pressure seal.
PACKING BOX: The chamber located in the BONNET which surrounds the
stem and contains the PACKING and other stem-sealing components.
PACKING FOLLOWER: A part that transfers a mechanical load to the
PACKING from the packing flange or nut.
PISTON ACTUATOR: A fluid-powered, normally pneumatic device in which
the fluid acts upon a movable cylindrical member, the piston, to provide
linear motion to the actuator stem. These units are spring or air opposed and
operate at higher supply pressures than a SPRING RETURN ACTUATOR.
PLUG: See CLOSURE MEMBER.
PORT-GUIDED: A valve plug that fits inside the seat ring, which acts as a
guide bushing. Examples: Splined Plug, Hollow Skirt, and the Feather-Guide
Plug.
POSITION SWITCH: A switch that is linked to the valve stem to detect a
single, preset valve stem position. Example: Full open or full closed. The
switch may be pneumatic, hydraulic, or electric.
POSITION TRANSMITTER: A device that is mechanically connected to the
valve stem and will generate and transmit either a pneumatic or electric
signal that represents the valve stem position.
POSITIONER: A device used to position a valve with regard to a signal. The
positioner compares the input signal with a mechanical feed back link from
the actuator. It then produces the force necessary to move the actuator
output until the mechanical output position feedback corresponds with the
pneumatic signal value. Positioners can also be used to modify the action of
the valve (reverse acting positioner), alter the stroke or controller input signal
(split range positioner), increase the pressure to the valve actuator
(amplifying positioner), or alter the control valve FLOW CHARACTERISTIC
(characterized positioner).
POST GUIDE: A guiding system where the valve stem is larger in the area
that comes into contact with the guide busings than in the adjacent stem
area.
PUSH-DOWN-TO-C LOSE: A term used to describe a LINEAR or GLOBE
STYLE valve that uses a DIRECT ACTING plug and stem arrangement. The
plug is located above the seat ring. When the plug is pushed down, the plug
contacts the seat, and the valve closes. Note! Most control valves are of this
type.
PUSH-DOWN-TO-OPEN: A term used to describe a LINEAR or GLOBE
STYLE valve that uses a REVERSE ACTION plug and stem arrangement.
The plug is located below the seat ring. When the plug is pushed down, the
plug moves away from the seat, and the valve opens.
PRESSURE RECOVERY FACTOR: See (F1).
QUICK OPENING: A FLOW CHARACTERISTIC that provides maximum
change in flow rate at low travels. The curve is basically linear through the
first 40% of travel. It then flattens out indicating little increase in flow rate as
travel approaches the wide open position. This decrease occurs when the
valve plug travel equals the flow area of the port. This normally happens
when the valve characteristics is used for on/off control.
RANGEABILITY: The range over which a control valve can control. It is the
ratio of the maximum to minimum controllable FLOW COEFFICIENTS. This
is also called TURNDOWN although technically it is not the same thing.
There are two types of rangeability - inherent and installed. Inherent
rangeability is a property of the valve alone and may be defined as the range
of flow coefficients between which the gain of the valve does not deviate
from a specified gain by some stated tolerance limit. Installed rangeability is
the range within which the deviation from a desired INSTALLED FLOW
CHARACTERISTIC does not exceed some stated tolerance limit.
REDUCED TRIM: Is an undersized orifice. Reduced or restricted capacity
trim is used for several reasons. (1) It adapts a valve large enough to handle
increased future flow requirement with trim capacity properly sized for
present needs. (2) A valve with adequate structural strength can be selected
and still retain reasonable travel vs. capacity relationships. (3) A valve with a
large body using restricted trim can be used to reduce inlet and outlet fluid
velocities. (4) It can eliminate the need for pipe reducers. (5) Errors in over
sizing can be corrected by use of restricted capacity trim.
REVERSE ACTING: This term has several deferent meanings depending
upon the device it is describing. A REVERSE-ACTING ACTUATOR is one in
which the actuator stem retracts with an increase in diaphragm pressure. A
REVERSE-ACTING VALVE is one with a PUSH-DOWN-TO-OPEN plug and
seat orientation. A REVERSE-ACTING POSITIONER or a REVERSE-
ACTING CONTROLLER outputs a decrease in signal in response to an
increase in set point.
REVERSE FLOW: Flow of fluid in the opposite direction from that normally
considered the standard direction. Some ROTARY VALVES are considered
to be bi-directional although working pressure drop capabilities may be lower
and leakage rates may be higher in reverse flow.
ROTARY VALVE: A valve style in which the FLOW CLOSURE MEMBER is
rotated in the flow stream to modify the amount of fluid passing through the
valve.
SEAT LOAD: The contact force between the seat and the valve plug. When
an actuator is selected for a given control valve, it must be able to generate
enough force to overcome static, stem, and dynamic unbalance with an
allowance made for seat load.
SEAT RING: A part of the flow passageway that is used in conjuction with
the CLOSURE MEMBER to modify the rate of flow through the valve.
SELF-CONTAINED REGULATOR: A valve with a positioning actuator using
a self-generated power signal for moving the closure member relative to the
valve port or ports in response and in proportion to the changes in energy of
the controlled variable. The force necessary to position the CLOSURE
MEMBER is derived from the fluid flowing through the valve.
SEPARABLE FLANGE: Also known as a SLIP-ON FLANGE. A flange that
fits over a valve body flow connection. It is generally held in place by means
of a retaining ring. This style of flange connection conforms to ANSI/ISA
275.20 and allows for the use of different body and flange materials.
Example: A valve with a stainless steel construction could use carbon steel
flanges. This type of valve is very popular in the chemical and petro-
chemical plants because it allows the use of exotic body materials and low
cost flanges.
SOFT SEATED: A term used to describe valve trim with an elastomeric or
plastic material used either in the VALVE PLUG or SEAT RING to provide
tight shutoff with a minimal amount of actuator force. A soft seated valve will
usually provide CLASS VI seat leakage capability.
SPLIT BODY: A valve whose body is split. This design allows for easy plug
and seat removal. Split-bodied valves are made in both the straight-through
and angle versions. The Masoneilan 2600 or ANNIN is an example of a split
body valve.
SPRING RATE: A term usually applied to SELF-CONTAINED
REGULATORS describing the range of set point adjustment available for a
particular range spring.
STATIC UNBALANCE: The net force produced on the valve stem by the
fluid pressure acting on the CLOSURE MEMBER and STEM within the
pressure retaining boundary. The closure member is at a stated opening
with a stated flow condition. This is one of the forces an actuator must
overcome.
STELLITE: Also called #6 Stellite or Alloy 6. A material used in valve trim
known for its hardness, wear and corrosion resistance. Stellite is available
as a casting, barstock material and may be applied to a softer material such
as 316 stainless steel by means of spray coating or welding.
STEM: The VALVE PLUG STEM is a rod extending through the bonnet
assembly to permit positioning of the plug or CLOSURE MEMBER. The
ACTUATOR STEM is a rod or shaft which connects to the valve stem and
transmits motion or force from the actuator to the valve.
STEM GUIDE: A guide bushing closely fitted to the valve stem and aligned
with the seat. Good stem guiding is essential to minimizing packing leakage.
SUPPLY PRESSURE: The pressure at the supply port of a device such as a
controller, positioner, or transducer. Common values of control valve supply
pressures are 20 psig. for a 3-15 psig. output and 35 psig. for a 6-30 psig.
output.
STROKE: See TRAVEL.
THROTTLING: Modulating control as opposed to ON/OFF control.
TRANSDUCER: An element or device which receives information in the form
of one quantity and coverts it to information in the form of the same or
another quantity. (See I/P)
TRAVEL: The distance the plug or stem moves in order to go from a full-
closed to a full-open position. Also called STROKE.
TRIM: Includes all the parts that are in flowing contact with the process fluid
except the body, BONNET, and body flanges and gaskets. The plug, seats,
stem, guides, bushings, and cage are some of the parts included in the term
trim.
TRUNNION MOUNTING: A style of mounting the disc or ball on the valve
shaft or stub shaft with two bushings diametrically opposed.
TURNDOWN: A term used to describe the ratio between the minimum and
maximum flow conditions seen in a particular system. Example: If the
minimum flow were 10 G.P.M. and the maximum flow were 100 G.P.M. the
turndown would be 10:1. This term is sometimes incorrectly applied to
valves. See RANGEABILITY.
VALVE: A device which dispenses, dissipates, or distributes energy in a
system.
VALVE BODY: See BODY.
VALVE FLOW COEFFICIENT: See Cv.
VALVE PLUG: See CLOSURE MEMBER.
VENA CONTRACTA: The location where cross-sectional area of the flow
stream is at its minimum size, where fluid velocity is at its highest level, and
where fluid pressure is at its lowest level. The vena contracta normally
occurs just downstream of the actual physical restriction in a control valve.

Commissioning of Instrumentation and Control System

Posted on October 7, 2007 by novakurniawan

Instrumentasi dan Kontrol pada dasarnya adalah ’sekedar’ alat pelengkap untuk menunjang
ketepatan pengukuran dan otomasi sistem dari alat-alat listrik, mesin, ataupun juga alat-
alat proses pengolahan atau produksi. Jika anda membayangkan contoh berikut misalnya, sebuah
oil refinery atau pengolahan minyak maka komponen utama yang akan terlihat di sana adalah
kolom-kolom destilasi, pipa-pipa, dan vesel-vesel untuk separasi. Mana instrument and
kontrolnya?. Instrumentasi hanyalah tempelan devices kecil-kecil di sekujur kolom destilasi,
perpipaan, vessel, dapat juga tank. Kontrol adalah seperangkat alat yang menerima informasi dan
memberikan perintah kepada instrument. Fungsinya adalah untuk melakukan otomasi atau
kontrol terhadap proses yang sedang berlangsung. Keberadaannya menjadi vital ketika akurasi
hasil, continuity proses, juga safety process menjadi pertimbangan utama dalam industri proses.

Dapat dikatakan bahwa instrumentasi dan kontrol berkembang sebagai cabang ilmu tersendiri
adalah baru saja pertengahan abad 20 setelah usai Perang Dunia ke-2. Sedangkan industri-
industri sudah berkembang satu abad sebelumnya.

Small but Important

Secara teori dalam kondisi equilibrium semua proses equipment, electric equipment, atau
mechanical equipment dapat dijalankan dengan baik. Kondisi equilibrium yang saya maksudkan
adalah suatu kondisi ideal atau kondisi set-point yang diperlukan agar suatu sistem dapat
berjalan dengan seimbang. Masalahnya adalah tidak selamanya kondisi yang diperlukan untuk
menjalankan suatu sistem itu ideal. Sehingga peran instrumentasi dan kontrol menjadi signifikan
dalam tahap ini untuk memenuhi semua kebutuhan kondisi set point tersebut.

Sebagai sebuah cerita pendek tentang Instrumentasi dan Kontrol:

Untu menjalankan turbin generator secara teori tidak ada sangkut pautnya dengan disiplin ilmu
Instrumentasi dan Kontrol. Dengan adanya bahan bakar yang akan memutar rotating equipment
(ilmu-nya mesin) maka engine dapat running. Ketika engine running maka ilmu elektrikal yang
menjelaskan bagaimana mengubah gerakan yang berpotongan dengan medan magnet menjadi
listrik. Ketika kita berhenti di sini maka sama sekali tidak terlihat instrumentasi dan kontrol
diperlukan di sana. Tetapi ternyata secara detail banyak sekali kebutuhan yang diperlukan oleh
turbine generator untuk running secara kontinu dan safe. Karena turbine generator memerlukan
kondisi ideal. Dia memerlukan supply bahan bakar yang stabil, memerlukan pressure lube oil
tertentu, memerlukan kepastian tidak ada flammable gas di enclosure-nya, memerlukan untuk
memindahkah kalor untuk menjaga suhunya, memastikan voltage dan current yang dihasilkan
tepat sesuai desain, dll. Semua kepastian tersebut hanya dapat dijamin dengan melakukan
pengukuran atau sensing variable dengan instrument. Dan jika kondisi ternyata tidak ideal maka
dapat dilakukan adjustment pada final element yang dapat di adjust dengan sistem kontrol untuk
mengubah sensing variabel. Nah, di situlah perlunya bidang ilmu yang satu ini.

Di dalam kegiatan commissioning peran dan fungsi dari sistem instrument dan kontrol adalah
sebagai penunjang continuity dan safety dari sistem yang sedang running. Oleh karena itu
monitoring, regular control agar tercapai continuity process, dan safety control untuk
keselamatan proses, sangat mutlak diperlukan.

Apa yang dilakukan commissioning engineer ketika commissioning sedang berjalan?

Ketika commissioning dilakukan berarti semua kegiatan pre-commissioning yang meliputi check
devices, check wiring, check control action sudah selesai dilakukan artinya semua devices ready
untuk beroperasi dalam kondisi normal dan kritikal.

Commissioning engineer menyiapkan: Operator Station, DCS/PLC specialis, Field Technician.

Operator Station bertugas untuk memonitor semua proses yang sedang berjalan.

DCS/PLC specialis bertugas untuk melakukan adjustment jika ditemukan abnormality pada
software atau logic.

Field Technician bertugas untuk stand by ready on job jika ditemukan abnormality pada field
devices.

Tugas seorang Instrument & Control Commissioning Engineer adalah ibarat “Kapten” (kalo suka
nonton aksi Kapten pada film-film tentang Kapal Selam, sang kapten berdiri dibelakang operator
yang sedang memantau monitor dan memberikan perintah-perintah). Sang Kapten memberikan
command-command kepada oparator, masukan untuk DCS specialist, dan command untuk field
technician serta menjaga komunikasi dengan engineer disiplin lain yang bertanggung jawab
terhadap sistem tersebut, bisa mekanikal, proses, piping, atau elektrik.

Jadi instrument dan kontrol pada dasarnya adalah melayani disiplin lain, namun sangat vital
diperlukan untuk menjamin proses berjalan secara continue dan safe.

Nova Kurniawan

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Control Valve Calibration

Posted on March 30, 2008 by novakurniawan

Control Valve adalah terminologi yang digunakan untuk suatu valve yang mempunyai
kemampuan throatling atau juga gradual changing. Apakah on-off valve termasuk controlled
valve? Iya, tetapi jarang sekali disebut sebagai control valve. Control valve terkhusus untuk
valve yang bisa menerima perintah analog baik dengan sinyal analogue maupun kumpulan sinyal
digital.

Kalibrasi control valve diperlukan untuk memastikan bahwa control valve dapat menghasilkan
respon aktuasi sebagaimana dikehendaki oleh sistem kontrol pada suatu proses. Respon aktuasi
yang dimaksud meliputi ketepatan pada value, linearity, dan juga respon time tentunya. Control
valve sebagai aktuator dalam suatu loop kontrol mempunyai peranan penting dalam
meregulating suatu proses. Kegagalannya dalam meregulating suatu proses adalah merupakan
indikasi abnormality suatu proses yang apabila berkelanjutan berefek kepada shutdown.

Ada 2 macam kalibrasi yang umum dikenal pada control valve yaitu Manual Calibration dan
Auto Calibration.

Manual calibration adalah kalibrasi dengan menggunakan input manual untuk control valve dan
sebagai pembanding adalah si pengkalibrasi. Inti dari pada kalibrasi adalah untuk membawa
value kepada nilai sebenarnya. Value dari suatu control valva adalah bukaan / opening. Bukaan
di value kan berupa percentage. Common sense mengatakan bahwa lima titik standar yang
dijadikan patokan sebagai opening control valve. 0%, 25%, 50%, 75%, 100%. Aktivitas kalibrasi
adalah untuk mengsinkronkan input kontrol valve yang berupa analogue signal (assumed HART)
dengan opening control valve. Nilai 4-20 mA sebagai standar instrumentasi direntangkan untuk
mewakili opening menjadi 4mA, 8mA, 12mA, 16mA, 20mA.

Dalam control valve dikenal terminologi Quick Opening, Linear, Equal Percentage. Istilah ini
untuk menunjukkan hubungan antara opening dan flow rate. Pertanyaan yang timbul adalah
apakah ketika Valve Quick Opening atau Equal Percentage maka opening travelnya tidak linear
dengan input signal? Menurut pendapat orang fisher bahwa Quick Opening, Linear, dan Equal
Percentage adalah trim characteristik yang sudah di set secara mechanical. Sehingga input signal
terhadap opening harus selalu linear. Pendapat tersebut menunjukkan term QO, Linear, dan
Equal% pada trim karakteristik.

Namun ada pertanyaan kembali kenapa dimungkinkan untuk mengganti input characteristik
menjadi Quick Opening, Linear, dan Equal Percentage dengan setting dari Handheld HART
Communicator 375?. Sehingga dihasilkan respon signal yang melengkung atau parabolik. Saya
belum menemukan jawaban yang spesifik tentang hal itu. Tapi requirement dari proses
memungkinkan untuk memberikan respon yang bersifat parabolik dari suatu control valve ketika
proses yang hendak dikontrol memang tidak linear. Sehingga dapat disimpulkan bahwa respon
kontrol dapat juga menjadi quick opening, linear, atau equal percentage.

Jarak travel juga menjadi hal penting yang perlu diperhatikan dalam kalibrasi. Jarak travel adalah
absolutely mechanical adjust pada stem control valve. Jarak travel adalah jarak dari fully open
sampai fully closed. Fully closed artinya sudah tidak dapat diadjust tapi untuk fully open
beberapa valve memberikan keleluasaan kepada user untuk memendekkan atau memanjangkan
travel. Reference utama dalam tahap konstruksi adalah bahwa jarak travel harus sesuai dengan
data sheet. Jika jarak travel salah maka control valve akan mendeteksi maksimum open sebagai
100%. Misalkan control valve anda jarak travelnya 3″, namun dalam kenyataannya secara
mekanikal berjarak 4″ maka dalam tahap kalibrasi nilai travel 4″ dianggap sebagai 100%,
padahal seharusnya kalau merefer kepada datasheet dengan travel 3″ maka nilai opening 4″
adalah sekitar 133%. Oleh karena itu harus benar-benar dipastikan bahwa jarak travel sudah
sesuai requirement pada datasheet dengan melakukan adjustment pada stem.

Autocalibration dapat dilakukan dengan menggunakan Handheld Fisher 375. Pilih menu
calibration and auto. Valve secara otomatik mencari highest postition dengan menstroke secara
penuh control valve, nilai itu akan secara otomatik dianggap sebagai nilai 100%. Kemudian
valve akan mencari nilai fully closed, dan nilai itu adalah 0%. And valve calibrated already.

Initial opening menjadi penting ketika tight shut off menjadi hal utama. Artinya ketika fully
closed 0% control valve harus benar-benar tight. Bagaimana memastikannya? Case untuk FC,
langkah pertama pastikan ketika feedback menunjukan 0%, kemudian release pressure-nya maka
control valve tidak ada gerakan turun lagi. Atau dengan mengirim signal dibawah nilai minimum
4 mA, misalnya 3.8 mA maka control valve juga sudah tidak dapat turun lagi. Kalo control
valve sudah terkalibrasi tetapi ketika dikirim signal 3.8 mA control valve masih turun lagi maka
nilai 0% belum sepenuhnya tight.

Hal yang perlu diperhatikan ketika melakukan kalibrasi adalah control valve dalam kondisi out
of service. Serta proteksi kalibrasi harus dihilangkan. Pastikan input karakteristik dan send ke
control valve. Setelah kalibrasi selesai control valve dikembalikan pada kondisi in-service.

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Instrumentation and Automation in Oil and Gas Industries

- Offshore
Posted on November 29, 2007 by novakurniawan

Oil and Gas Offshore industry lebih dari setengah abad telah menjadi penopang kebutuhan
energi dunia setelah puluhan tahun sebelumnya onshore industri lebih mendominasi industri ini.
Salah satu bidang disiplin ilmu yang tidak dapat terlepas keberadaannya dalam menunjang
kebutuhan energi dunia ini untuk offshore industri adalah instrumentasi dan kontrol
(automation). Disiplin ilmu ini berperan dalam memberikan monitoring dan otomatisasi atau
kontrol pada proses secara normal kontinyu dan dan juga berperan menjamin safety pada kondisi
hazard atau bahaya dari dalam proses itu sendiri juga kebakaran. Sebagaimana harus dipahami
bahwa pengolahan oil and gas pada dasarnya adalah industri proses yang artinya adalah kegiatan
hilir. Kegiatan hulu explorasi dan exploitasi ataupun pengeboran dari titik 0 sampai negatif
kebawah permukaan tanah atau air laut diluar dari pembahasan website ini.

Sampai sejauh mana scope disiplin ilmu ini digunakan dalam dunia Oil & Gas Offshore, berikut
ini adalah sebuah cerita pendek tentangnya.

Study Case Arthit Process Platform PTT E&P

Instrumentasi dan Otomasi secara hierarki dibagi menjadi dua system besar yaitu Process
Control System dan Safety Instrumented System. Process Control System (PCS) merupakan
sistem penunjang proses monitoring, regulatory control, atau pun juga advance regulatory
control. Safety Instrumented System (SIS) merupakan sistem yang diaplikasikan untuk menjamin
safety yang meliputi Emergency Shutdown System (ESD) dan Fire & Gas System (F&G).Ketika
normal operasi maka fungsi kontrol reguler akan dijalankan oleh PCS secara kontinyu.
Monitoring open loop, running simple closed loop antara tranmitter dan control valve yang
bekerja mencari kestabilan performance kontrolnya adalah fungsi-fungsi sederhana dari PCS.
Demikian juga halnya dengan cascade loop, ratio, split range, yang semuanya beroperasi untuk
mencari kestabilan sepenuhnya juga ditangani oleh PCS. Numerik control calculation untuk
simple closed loop, cascade, ratio, split range masih dipercayakan kepada iterasi jitu PID.

Sesuai dengan perkembangan teknologi yang mengutamakan efisiensi maka FIELDBUS

FOUNDATION dipilih menjadi alternatif bagi distribusi pendelegasian sistem kontrolnya.
Banyak fabrikan yang sudah mensenjatai devicenya dengan fieldbus, salah satunya adalah
Yokogawa. Transmitter fieldbus yang digunakan pada Arthit CPP Platform Gulf of Thailand
adalah Yokogawa EJA, control processingnya atau controllernya dipercayakan kepada Yokogawa
Fieldbus CS 3000, dan Control Valve masih dipercayakan kepada Fieldvue Fisher Rosemount
CCI. Meskipun sistem PCS pada platform ini merupakan campuran dari berbagai manufacture
tetapi karena setiap device sudah dipersenjatai memakai approved protokol yaitu Foundation
Fieldbus, maka tidak perlu diragukan lagi compatibilty-nya dalam menunjang sistem kontrol.

Safety system yang lebih dikenal dengan Safety Instrumented System (detail definisi dapat dbaca
artikel lain di page ini -pen), terdiri atas ESD system dan F&G system. Keduanya saling
berhubungan dan berinteraksi. ESD system menjalankan fungsi process safety hazard. Dalam
P&ID dengan mudah SIS dapat diidentifikasi sebagai sebuah kondisi proses yang Hi-Hi atau Lo-
Lo. Demikian halnya F&G system akan menjalankan fungsi ketika adanya gas atau fire yang
timbul di area platform. Kritikal process menghasilkan effect shutdown demikian juga halnya
kehadiran api atau gas berbahaya akan menghasilkan inisiasi shutdown sesuai dengan levelnya
menurut cause and effect matrix. Untuk safety process, transmitter yang digunakan masih dari
produk Yokogawa EJA dengan HART protocol, control system menggunakan Yokogawa Prosafe,
sedangkan final element adalah simple digital output untuk solenoid dan atau relay. F&G system
masih mengandalkan produk dari Detronic untuk gas dan flame detector, VESDA untuk High
Sensitivity Smoke Detector, Manual Alarm Call Point dan ESD button menggunakan produk
MEDC.

Demikian sekilas gambaran scope instrumentasi dan kontrol untuk offshore platform oil & gas
facility.

Nova Kurniawan

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Loop Testing / Pre-commissioning

Posted on October 3, 2007 by novakurniawan

Definisi:

Loop adalah sistem yang terintegrasi untuk menggabungkan instrument field device dari dan ke lapangan
dengan sistem control yang jejaringnya dihubungkan dengan wiring. Open Loop adalah hubungan signal
satu arah dari lapangan (field devices) ke sistem kontrol atau sebaliknya dari sistem kontrol ke lapangan.
Closed Loop adalah siklus signal dari lapangan (field devices) ke sistem kontrol kemudian diolah dan
menghasilkan signal output yang akan dikirimkan kembali ke lapangan (final element). Single Loop
adalah loop sederhana dapat berupa open loop atau closed loop sederhana. Pre-Commissioning adalah
aktivitas untuk memastikan bahwa setiap instrument input devices, sistem control, dan instrument final
element dapat beroperasi sebagaimana control design untuk menunjang proses. Pre-commissioning
dapat juga disebut function test untuk setiap loop. Commissioning adalah aktivitas untuk membuat suatu
sistem ‘LIVE’ dan beroperasi secara normal. Commissioning melibatkan multidisiplin dan multi loop.
Instrument dan control sistem harus dapat berfungsi normal untuk mengontrol sistem multidisiplin
(proses, electrical, mechanical) yang sedang berjalan dan juga menjaga safety protection systemnya
dalam kondisi kritikal. Contoh: Instrument Air System, Separation System, Power Generation System,
Heating Medium System, dll. Start-Up adalah aktivitas untuk menghidupkan semua sistem yang
menunjang beroperasinya plant secara keseluruhan. Seluruh field device input, logic control, final
element untuk berbagai sistem pada seluruh plant harus beroperasi dan terkontrol sempurna
sebagaimana desain proses.Contoh: Start-Up semua system yang sudah pernah dicommissioning
dengan sequence sesuai dengan requirement process or utility. Jadi pada dasarnya Pre-Commissioning
untuk Instrument adalah Loop Check dan langkah-langkahnya adalah sebagai berikut:

Loop Test untuk SMART Transmitter:

1. Memastikan bahwa Hook-Up installation sudah benar.

2. Memastikan bahwa wiring terminasi sudah benar dengan melakukan cold wire test or continuity
test end to end dari field device sampai marshalling panel.
3. Memastikan polarity sudah benar.
4. Menyambungkan knife switch di marshalling panel.
5. Instrument harus terenergize, pastikan dari indicator atau check voltage-nya.
6. Melakukan Trim yaitu check ZERO and SPAN dengan screw adjustment.
7. Monitor di Engineering Work Station value yang dikirim dari lapangan harus sesuai ZERO dan
SPAN-nya.
8. Melakukan injeksi proses (pressure, level, temperature) pada transmitter tapping point dan verify
indikator lokal serta verify signal yang dikirim ke Engineering Work Station.
9. Melakukan linearisasi 0%, 25%, 50%, 75%, 100%.
10. Loop Test Complete.
Loop Test untuk SMART Control Valve adalah:

1. Melakukan langkah-langkah seperti point 1- 4 seperti pada SMART transmitter.

2. Check voltage pada terminal control valve.
3. Pastikan pada saat ZERO signal control valve fully closed.
4. Manual injeksi sinyal dari Engineering Work Station 0%, 25%, 50%, 75%, 100% dan Control
Valve harus menunjukkan nilai travel linear yang sama. Apabila tidak sama maka lakukan AUTO-
CALIBRATION.
5. Check kondisi FAIL POSITION dengan melepas kabel atau mengisolate air supply.
6. Untuk close loop lakukan setting AUTO pada faceplate workstation.
7. Check control action REVERSE or DIRECT.
8. Loop Test Complete

Loop Test untuk FIELDBUS Transmitter adalah:

1. Melakukan langkah-langkah seperti point 1- 5 pada SMART transmitter.

2. Request Control Engineer or Software Specialist untuk melakukan Fieldbus configuration dan
lakukan download.
3. Melakukan langkah-langkah seperti pada point 6-10 pada SMART transmitter.

Loop Test untuk FIELDBUS Control Valve adalah:

1. Melakukan langkah-langkah seperti point 1- 2 pada SMART Control Valve.

2. Request Control Engineer or Software Specialist untuk melakukan Fieldbus configuration dan
lakukan download.
3. Melakukan langkah-langkah seperti point 3- 8 pada SMART Control Valve.

Nova Kurniawan

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Temperature Instrument
Posted on January 5, 2008 by novakurniawan

Thermocouple adalah dua logam yang didekatkan yang apabila terpapar oleh kalor dengan suhu
tertentu akan menghasilkan beda potensial or mV yang sebanding dengan perubahan temperature
(Seebeck Effect). RTD adalah resistance temperature detector, sensor ini akan menghasilkan
perubahan resistance seiring dengan perubahan temperature. Kedua hal di atas adalah sensor-
sensor yang umum digunakan di industri oil and gas untuk mengukur temperature. Kedua
besaran di atas yaitu mV dan Ohm akan dilinierisasi dan diconvert menjadi 4-20 mA oleh
transmitter. Bagaimana menkonvert perubahan mV menjadi 4-20 mA? dan bagaimana
mengkonvert perubahan besaran Ohm menjadi 4-20 mA? Silahkan anda membuat rangkain
untuk itu dengan sumber tegangan tetap sebesar 24 VDC.

Ada banyak type sensor RTD dan TC namun yang biasa saya temukan dalam pekerjaan saya
adalah Pt-100 (RTD) dan Type-K (TC). Hitungan matematika yang biasa digunakan untuk
menghubungkan temperature dengan mV dan Ohm biasanya hanya tertinggal di meja teori.
Praktisnya anda harus memiliki Tabel Standar yang menunjukkan hubungan mV dan Ohm
dengan temperature. Umumnya teknisi bekerja berdasarkan tabel itu.

Bagaimana melakukan kalibrasi?

Bench Calibration dilakukan di dalam shop dengan membandingkan dengan temperature gauge
standar. Dahulu kala dikenal istilah sand bath atau oil bath yang bisa dipanaskan. Temperature
Gauge standar dimasukkan ke dalam bath bersama dengan temperature instrument yang hendak
dikalibrasi. Increase and decrease temperature pada sand bath sehingga didapatkan akurasi value
dan linearity pada temperature instrument. Sekarang dikenal istilah Jofra (Trade Mark) yang
menggantikan sand bath sebagai tungku pemanas. Jofra dengan electricity power memiliki
element pemanas yang akan menghasilkan controlled temperature. Controlled temperature ini
yang akan digunakan sebagai referensi untuk melakukan kalibrasi.

Ambient temperature adalah juga tolok ukur yang penting untuk mengecek apakah sebuah
temperature instrument bekerja atau tidak. Ketika temperature instrument dalam kondisi non
operasi tidak menunjukkan ambient maka tanda-tanda bahwa ada masalah dengan instrument
tersebut.

Pre-commissioning / Function and Loop Test

Function test dilakukan On-site setelah temperature instrument hooked up. Langkah awal dari
loop test tentu saja adalah wiring test or continuity and polarity test. Kemudian plug-in knife
switch, temperature instrument harus menunjukkan nilai ambient temperature pada display.
Namun jika display tidak menyala hendaknya dire-check wiring, polarity dan power supply ke
instrument. Demikian halnya di operating station harus menunjukkan kalau temperature
instrument healthy. Jika masih terdapat status IOP maka abnormality harus segera direpair.
Troubleshooting adalah kegiatan yang harus dilakukan per tahap. Sampai cara paling akhir
adalah dengan komparasi interchange modul, hanya untuk membuktikan bahwa modul
transmitter masih baik atau sudah rusak. Ketika ditemukan modul transmitter rusak, tahap
commissioning hanya bisa report & klaim. Repair untuk modul transmitter diperlukan expertize
khusus. Bahkan mungkin seorang maintenance engineer lebih mengerti bagaimana untuk
melakukan repair.

Tahap simulasi on site pada tahap pre-commissioning ditujukan kepada check transmitter.
Karena injeksi signal dilakukan dari input transmitter. Dalam hal ini signal injector berfiungsi
untuk menggantikan elemen sensor. RTD type sensor dapat disimulasikan dengan decade
resistance box dan Thermocouple type sensor dapat disimulasikan dengan mV injector. Kenaikan
temperatur yang linear tidak akan menghasilkan sensor signal yang linear tapi cenderung
logaritmik. Oleh karena itu fungsi transmitter untuk kembali melakukan linearisasi dengan
membalik fungsi logaritmik tersebut.

Masih banyak aspek yang bisa dipelajari tentang temperature instrument. Tapi keterbatasan
pengetahuan murni penulis, artinya tidak copy paste atau mencomot dari tulisan lain, membuat
tulisan ini menjadi pendek. Seiring berjalannya waktu dan bertambahnya pengalaman semoga
tulisan ini makin lengkap.

Nova Kurniawan